index.css

.menu {
  opacity: 0.0;
  transform: translate3d(-100%, 0, 0);
  transition: opacity 2s ease transform 2s ease;
}

.menu.is-open {
  opacity: 1.0;
  transform: translate3d(0, 0, 0);
  transition: opacity 2s ease transform 2s ease;
}

index.jsx

import React, { Component, PropTypes } from 'react';
import cx from 'classnames';

export default class MyComponent extends Component {
  render() {
    // any classes that should always be on just use true as value
    const menuClasses = cx({
      'menu': true,
      'is-open': this.state.isOpen
    });
    
    return (
      <div className={ menuClasses }>
        Foo
      </div>
    );
  };
}